This invention relates generally to powder metal friction materials, and more particularly to iron-base, sintered powder metal friction material suitable for use as railroad brake shoes. For many years, cast iron has been used as a brake shoe material for tread braking railroad vehicles. In an effort to improve wear life of railroad brake shoes, composition shoes were developed.
With the use of a composition shoe, the coefficient of friction can be tailored to the specific application requirements. However, some composition shoes are subject to wet fade, a condition which causes elongated stopping distances in wet weather. Also, composition shoes have been found to have a very short life when used in high energy braking applications, such as on locomotives, especially switching locomotives.
Another problem associated with composition shoes is that the products of decomposition, or debris, especially graphite, tend to "pollute" the wheel and rails, thus reducing their coefficient of friction.
Another problem with some high friction composition shoes has been their tendency to exhibit metal pickup, a phenomenon wherein slivers of wheel steel are transferred to the composition shoes. The metal on the shoes then causes gouging of the wheels during subsequent braking. Such gouging will cause wheels to quickly go out of tolerance and produce high flanges. These problems require premature wheel replacement and consequently increase costs.
Because of the problems with state-of-the-art railroad brake shoes, it was felt that a new type of railroad brake shoe should be developed. It was thought that a sintered powder metal material might be suitable, since it would probably exhibit very little wet fade.
Sintered powder metal shoes have been developed for use in aircraft disc braking. Although aircraft braking is also high-energy braking, it is otherwise quite different from railroad braking. Aircraft braking is normally disc braking, while railroad braking is usually accomplished by engaging brake shoes with the running surface or tread of the cast steel wheels themselves. Because of this, aircraft friction materials have always been quite different from railroad friction materials. Sintered powder metal friction materials have also been recently introduced into the automotive market as disc pads. Automotive disc brakes are quite different from railroad tread brakes because of the energy levels involved and a single type of friction material is not suitable for both applications.